Range extension techniques for a wireless local area network

ABSTRACT

Techniques for extending transmission range in a WLAN are described. In an aspect, a receiving station determines the frequency error between a transmitting station and the receiving station based on one or more initial packet transmissions and corrects this frequency error for subsequent packet transmissions received from the transmitting station. The residual frequency error is small after correcting for the frequency error and allows the receiving station to perform coherent accumulation/integration over a longer time interval to detect for a packet transmission. The longer coherent accumulation interval improves detection performance, especially at low SNRs for extended transmission range. The techniques may be used whenever the receiving station knows the identity of the transmitting station, e.g., if the subsequent packet transmissions are scheduled. In another aspect, a preamble is generated with a longer spreading sequence and sent with each packet transmission.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application claims priority to provisional U.S. ApplicationSer. No. 60/750,183, entitled “RANGE EXTENSION TECHNIQUES FOR A WIRELESSLOCAL AREA NETWORK,” filed Dec. 13, 2005, assigned to the assigneehereof and incorporated herein by reference.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for extending transmission range for awireless local area network (WLAN).

II. Background

Wireless communication networks are widely deployed to provide variouscommunication services such as data, voice, video, and so on. Thesewireless networks include wireless wide area networks (WWANs) thatprovide communication coverage for large geographic areas (e.g.,cities), wireless local area networks (WLANs) that provide communicationcoverage for medium-size geographic areas (e.g., buildings), andwireless personal area networks (WPANs) that provide communicationcoverage for small geographic areas (e.g., homes).

IEEE 802.11 is a family of standards developed by The Institute ofElectrical and Electronics Engineers (IEEE) for WLANs. These standardscover medium range radio technologies. IEEE Std 802.11 1999 Edition (orsimply, “802.11”) supports data rates of 1 and 2 mega bits/second (Mbps)in the 2.4 giga Hertz (GHz) frequency band using frequency hoppingspread spectrum (FHSS) and direct sequence spread spectrum (DSSS). IEEEStd 802.11a-1999 (or simply, “802.11a”) supports data rates of 6 to 54Mbps in the 5 GHz frequency band using orthogonal frequency divisionmultiplexing (OFDM). IEEE Std 802.11b-1999 (or simply, “802.11b”)supports data rates of 1 to 11 Mbps in the 2.4 GHz band using DSSS. IEEEStd 802.11g-2003 (or simply, “802.11g”) supports data rates of 1 to 54Mbps in the 2.4 GHz band using DSSS and OFDM. These various IEEE 802.11standards are known in the art and publicly available.

The lowest data rate supported by the IEEE 802.11 standards is 1 Mbps. Acertain minimum signal-to-noise-and-interference ratio (SNR) is requiredfor reliable reception of a transmission sent at the lowest data rate of1 Mbps. The range of the transmission is then determined by thegeographic area within which a receiving station can achieve therequired SNR or better. In certain instances, it is desirable to send atransmission with a range that is greater than the range for the lowestdata rate supported by the IEEE 802.11 standards. Furthermore, it isdesirable to achieve the greater transmission range with minimumincrease in hardware complexity at both the transmitting and receivingstations.

There is therefore a need in the art for cost-effective techniques toextend the transmission range for a WLAN.

SUMMARY

Techniques for extending transmission range in a WLAN are describedherein. In an aspect, a receiving station determines the frequency errorbetween a transmitting station and the receiving station based on one ormore initial packet transmissions and corrects this frequency error forsubsequent packet transmissions received from the transmitting station.A packet transmission is a transmission of some amount of data withinsome amount of time. The residual frequency error is small aftercorrecting for the frequency error and allows the receiving station toperform coherent accumulation/integration over a longer time interval todetect for a packet transmission. The longer coherent accumulationinterval improves detection performance, especially at low SNRs that maybe encountered for extended transmission range. The frequency correctiontechniques may be used whenever the receiving station knows the identityof the transmitting station, which may be the case, e.g., if thesubsequent packet transmissions are scheduled.

In another aspect, a preamble is generated with a longer spreadingsequence and sent with each packet transmission. The receiving stationmay perform coherent accumulation over the length of the longerspreading sequence to achieve more reliable detection of the preamble atlow SNRs.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a WLAN with an access point and multiple user terminals.

FIG. 2 shows a transmission timeline for the WLAN.

FIG. 3 shows a packet and a preamble for 802.11b.

FIG. 4 shows a process for receiving data with frequency correction.

FIG. 5 shows an apparatus for receiving data with frequency correction.

FIG. 6 shows a preamble for a range extension mode.

FIG. 7 shows a block diagram of a transmitting station and a receivingstation.

FIG. 8 shows an embodiment of an acquisition processor.

FIGS. 9A through 9C show another embodiment of the acquisitionprocessor.

FIG. 10 shows a block diagram of a frequency error estimator.

FIG. 11 shows a process for receiving data in the WLAN.

FIG. 12 shows an apparatus for receiving data in the WLAN.

DETAILED DESCRIPTION

The range extension techniques described herein may be used for variousradio technologies and standards, such as IEEE 802.11. For clarity, muchof the following description is for 802.11b and 802.11g, which iscommonly referred to as 802.11b/g.

FIG. 1 shows a WLAN 100 with an access point 110 and multiple userterminals 120. An access point is a station that communicates with theuser terminals. An access point may also be called, and may contain someor all of the functionality of, a base station, a base transceiversubsystem (BTS), a Node B, and/or some other network entity. Userterminals 120 may be distributed throughout WLAN 100, and each userterminal may be fixed or mobile. A user terminal may also be called, andmay contain some or all of the functionality of, a mobile station, auser equipment (UE), and/or some other device. A user terminal may be awireless device, a cellular phone, a laptop computer, a personal digitalassistant (PDA), a wireless modem card, and so on. A user terminal maycommunicate with an access point or another user terminal.

For a centralized network architecture, a network controller 130 couplesto the access points and provides coordination and control for theseaccess points. Network controller 130 may be a single network entity ora collection of network entities. For a distributed architecture, theaccess points may communicate with one another as needed without theuses of network controller 130.

In general, a WLAN may include any number of stations, where a station(STA) may be an access point or a user terminal. A station may implementany one or any combination of IEEE 802.11 standards, e.g., 802.11band/or 802.11g. A station may also implement a range extension mode thatsupports at least one data rate that is lower than 1 Mbps. For example,the range extension mode may support data rates of 500 kilo bits/second(Kbps), 250 Kbps, 125 Kbps, and so on, or a combination of these lowerdata rates. In general, packet transmissions at progressively lower datarates may be received at progressively lower SNRs, which may be achievedfor progressively larger geographic areas. This is because a certainminimum bit-energy to noise-density (Eb/No) is typically required forreliable reception of a packet transmission. Hence, as the data ratedecreases, a data bit is transmitted over a longer time duration, therequired signal level at the receiving station is reduced, and thetransmission range is increased. For example, a packet transmission at125 Kbps may be reliably received at an SNR that is much lower than therequired SNR for 1 Mbps. Hence, the 125 Kbps transmission has a longertransmission range and a greater coverage area than the 1 Mbpstransmission.

FIG. 2 shows an example transmission timeline 200 for WLAN 100. Accesspoint 110 maintains a timeline for all transmissions covered by theaccess point. Access point 110 periodically transmits a beacon thatcarries (1) a preamble used by other stations for acquisition and (2)various types of information used to support communication with theaccess point. The information in the beacon includes (1) an access pointidentifier (AP ID) that allows the user terminals to detect and identifythe access point and (2) a beacon interval that indicates the timeperiod between consecutive beacon transmissions. The beacon istransmitted at target beacon transmit times (TBTTs), which are spacedapart by the beacon interval.

The time period between TBTTs may be divided into a contention freeperiod (CFP) and a contention period (CP). The contention free periodcovers the beacon as well as other transmissions that are controlled orscheduled by the access point. Hence, only one station transmits on thewireless medium at any given moment during the contention free periodand there is no contention among the stations for the wireless mediumduring this period. The contention period covers transmissions that arescheduled by the access point as well as transmissions that are notscheduled by the access point. Hence, more than one station may transmitsimultaneously on the wireless medium during the contention period.

IEEE 802.11 specifies three channel access functions, which are called adistributed coordination function (DCF), a point coordination function(PCF), and a hybrid coordination function (HCF). The DCF supportscontention-based channel access via a carrier sense multiple access withcollision avoidance (CSMA/CA) protocol. For the DCF, packettransmissions are not scheduled, and a station may transmit if it sensesthat the wireless medium is not busy. The DCF is operational during thecontention period.

The PCF supports contention-free channel access via a centralized pointcoordinator that is implemented at the access point. For the PCF, thepoint coordinator polls specific stations for transmission, and astation may transmit only if it is polled. The point coordinator mayalso transmit data to specific stations. The PCF is operational duringthe contention free period.

The HCF supports (1) an enhanced distributed channel access (EDCA),which is a contention-based channel access scheme, and (2) anHCF-controlled channel access (HCCA), which is a contention-free channelaccess scheme that is controlled by a hybrid coordinator. Packettransmissions are not scheduled for the EDCA and are scheduled for theHCCA. The EDCA is used during the contention period, and the HCCA may beused in the contention period or the contention free period. The HCF,EDCA and HCCA are described in IEEE Std 802.11e (or simply, “802.11e”).

In general, a packet transmission in a WLAN may be scheduled orunscheduled. A station may receive an unscheduled packet transmission atany time from another station during the contention period and typicallydoes not know the identity of the transmitting station until after thepacket has been detected. A station may receive a scheduled packettransmission from another station at a specific time instant or within atime window and typically knows the identity of the transmitting stationprior to receiving the packet transmission.

WLAN stations are typically designed to receive unscheduled packettransmissions. Since the stations within a WLAN typically operatewithout locking their clocks to a common reference frequency, eachstation normally performs acquisition independently for each receivedpacket transmission. Acquisition normally entails detecting for thepresence of a packet transmission and determining the timing andfrequency of the detected packet transmission. Acquisition is oftenachieved based on a preamble that is sent with each packet in IEEE802.11.

For IEEE 802.11, traffic data is processed by a medium access control(MAC) layer as MAC protocol data units (MPDUs). Each MPDU is processedby a physical layer convergence protocol (PLCP) and encapsulated in aPLCP protocol data unit (PPDU). Each PPDU is further processed by aphysical layer and transmitted via the wireless medium. A PPDU is oftenreferred to as a packet.

FIG. 3 shows a PPDU format for 802.11b/g. A PPDU includes a PLCPpreamble, a PLCP header, and an MPDU. The PLCP preamble includes a PLCPsynchronization (SYNC) field and a start frame delimiter (SFD) field.The SYNC field carries a fixed 128-bit sequence that is often referredto as a preamble. The SFD field carries a fixed 16-bit sequence thatindicates the start of the PLCP header. The PLCP header includes variousfields that convey the data rate, duration, and other information forthe MPDU. The MPDU carries traffic data and has a variable length. ThePLCP preamble and PLCP header are sent at 1 Mbps. The PLCP preamblecontains a total of 144 bits, which are processed to generate 144 BPSKsymbols. These 144 BPSK symbols are transmitted in 144 symbol periods,with each symbol period having a duration of 1 microsecond (μs).

FIG. 3 also shows the preamble for 802.11b. This preamble is composed ofa known sequence of 128 pilot bits that is generated based on apseudo-random number (PN) generator. The 128 pilot bits are denoted asd₀ through d₁₂₇. Each pilot bit is spread with an 11-chip spreadingsequence of {+1, −1, +1, +1, −1, +1, +1, +1, −1, −1, −1}, which iscalled a Barker sequence. This preamble has a length of 128 μs.

A receiving station generates input samples for a received signal andcorrelates the input samples with the 128-bit pilot sequence and the11-chip Baker sequence to detect for the presence of a preamble. Thereceiving station may perform coherent accumulation and non-coherentaccumulation for preamble detection. Coherent accumulation refers to theaccumulation, integration or sum of complex values, where the phases ofthe complex values affect the accumulation result. Non-coherentaccumulation refers to the accumulation, integration or sum of realvalues, e.g., magnitudes. Preamble detection is described in detailbelow.

Detection performance is dependent on various factors such as SNR andfrequency error/offset. For a packet received at a high SNR, a preamblemay be easily detected by coherently accumulating over a small number ofchips of the preamble. For a packet received at a low SNR, coherentaccumulation over more chips may be required for reliable detection ofthe preamble.

Frequency error between the transmitting and receiving stationsdetermines the number of chips that may be coherently accumulatedwithout incurring significant combining loss. A large frequency errorresults in a large phase shift across the preamble. For example, afrequency error of ±40 parts per million (ppm) at 5.8 GHz corresponds toa phase shift of ±83° over the 11-chip Barker sequence. Frequency errorthus results in the input samples being progressively more out of phaseacross the preamble and hence limits the number of chips that may becoherently accumulated.

A receiving station typically performs coherent accumulation over the11-chip Barker sequence and non-coherent accumulation over the 128-bitpilot sequence. This scheme provides good detection performance for theworst-case frequency error and the required SNR for 1 Mbps, which is thelowest data rate supported by IEEE 802.11.

A receiving station may observe a low SNR for a low rate (e.g., 125Kbps) packet transmission in the range extension mode. The receivingstation may perform coherent accumulation over more than 11 chips (e.g.,over 44 chips) in order to achieve good detection performance. Thecoherent accumulation interval may be selected based on the required SNRfor the packet transmission, which is lower for the range extensionmode. To account for more phase shift across a longer coherentaccumulation interval, the receiving station may perform detection formultiple frequency hypotheses. Each frequency hypothesis corresponds toa different hypothesized frequency error between the transmitting andreceiving stations. The frequency hypotheses may be selected such thatcoherent accumulation may be performed over the interval selected forthe required SNR. In general, the coherent accumulation interval islimited by frequency offset, and coherent accumulation may be performedover a longer interval for lower frequency offset. For each frequencyhypothesis, the receiving station may remove the hypothesized frequencyerror prior to performing coherent accumulation. The correct frequencyhypothesis that comes closest to the actual frequency error will causethe smallest amount of phase shift across the coherent accumulationinterval and provide the largest accumulated result.

The receiving station may perform preamble detection for multiplefrequency hypotheses in order to achieve good detection performance atlow SNRs with independent acquisition on each packet transmission. Theacquisition hardware may be replicated multiple times in order toevaluate the multiple frequency hypotheses simultaneously. Eachacquisition hardware may be each tuned to a different frequencyhypothesis and may perform coherent accumulation over the selectedinterval. The acquisition hardware with the largest deflection statisticwill be the one with the lowest frequency error. However, the replicatedhardware may significantly increase the cost of the station, which isundesirable.

In an aspect, a receiving station determines the frequency error betweena transmitting station and the receiving station based on one or moreinitial packet transmissions and corrects this frequency error forsubsequent packet transmissions received from the transmitting station.The frequency correction techniques may be used whenever the receivingstation knows the identity of the transmitting station. The receivingstation may expect a number of packet transmissions from the sametransmitting station under various operating scenarios such as, e.g., avoice over IP (VoIP) call, a large file transfer, and so on. In general,the expected packet transmissions may or may not be scheduled. Thereceiving station may perform acquisition for the expected packettransmissions after correcting or removing the known frequency error forthe transmitting station. The residual frequency error is small aftercorrecting for the known frequency error and allows the receivingstation to perform coherent accumulation over a longer interval, whichimproves detection performance. In essence, the receiving station canperform acquisition for each expected packet transmission with a singlecorrect frequency hypothesis.

The receiving station may perform initial acquisition using limitedacquisition hardware. For example, the acquisition hardware may be ableto evaluate only one frequency hypothesis for each packet transmission.In this case, the receiving station may perform acquisition with adifferent frequency hypothesis for each packet transmission. Thereceiving station can detect a packet transmission with high probabilitywhen the correct frequency hypothesis is selected. The receiving stationmay miss one or more packet transmissions before a preamble is detected.The missed packet transmissions may have a small impact on the overallperformance since these packet transmissions may simply be for callsetup and/or may be retransmitted.

FIG. 4 shows an embodiment of a process 400 performed by a receivingstation to receive data with frequency correction. The receiving stationreceives at least one initial packet transmission from a transmittingstation in a WLAN (block 412). The receiving station performs detectionfor the at least one initial packet transmission (block 414). Dependingon its hardware capability, the receiving station may perform detectionfor each packet transmission with one or multiple frequency hypotheses.The receiving station determines the frequency error between thetransmitting and receiving stations based on a detected packettransmission, which is typically the last initial packet transmission(block 416). The receiving station then performs detection for at leastone subsequent packet transmission from the transmitting station withthe frequency error corrected (block 418). The frequency errorcorrection may be achieved by (1) adjusting the frequency of adownconversion local oscillator (LO) signal at the receiving stationand/or (2) digitally rotating the input samples, as described below.

The initial and subsequent packet transmissions may be sent at a datarate that is lower than 1 Mbps for the range extension mode. The packettransmissions may also be sent at a data rate supported by IEEE 802.11.The subsequent packet transmissions may or may not be scheduled.

The receiving station may perform coherent accumulation over more than11 chips to detect a packet transmission. The receiving station may usethe same or different coherent accumulation intervals for the initialand subsequent packet transmissions. For example, the receiving stationmay perform coherent accumulation over (1) a first interval for initialacquisition when the identity of the transmitting station is not knownand (2) a second interval that is longer than the first interval forsubsequent acquisition when the identity of the transmitting station isknown. A short interval may be used for initial acquisition to accountfor channel fades, unknown frequency error, etc. A longer interval maybe used for subsequent acquisition for more reliable detection.

FIG. 5 shows an embodiment of an apparatus 500 for receiving data withfrequency correction. Apparatus 500 includes at least one processor 512for receiving at least one initial packet transmission from atransmitting station in a WLAN, at least one processor 514 forperforming detection for the at least one initial packet transmission,at least one processor 516 for determining the frequency error betweenthe transmitting and receiving stations based on a detected packettransmission, and at least one processor 518 for performing detectionfor at least one subsequent packet transmission from the transmittingstation with the frequency error corrected.

In another aspect, a preamble is generated with a spreading sequencethat is longer than 11 chips. A receiving station may perform coherentaccumulation over the length of the longer spreading sequence to achievemore reliable detection of the preamble at low SNRs, which may beencountered in the range extension mode. The preamble may also beextended longer to also improve detection performance.

FIG. 6 shows an embodiment of a preamble 600 that may be used for therange extension mode. For this embodiment, the preamble is composed of asequence of 64 pilot bits that may be generated based on a PN generator.The 64 pilot bits are denoted as d₀ through d₆₃. For this embodiment,each pilot bit d_(i), for i=0, . . . , 63, is spread with anintermediate sequence of four binary values {+1, +1, −1 and +1}, andeach binary value is further spread with the 11-chip Barker sequence.Each pilot bit is thus spread with a 44-chip spreading sequence that iscomposed of four instances of the 11-chip Barker sequence, with thethird instance of the Barker sequence being inverted in polarityrelative to the other three instances of the Barker sequence. Thispreamble has a length of 256 μs and can provide reliable detection fordata rates down to 125 Kbps.

For the embodiment shown in FIG. 6, a receiving station may performcoherent accumulation over the 44-chip spreading sequence and mayperform non-coherent accumulation over the 64-bit pilot sequence. ForIEEE 802.11, the maximum frequency error is ±40 ppm, which correspondsto ±232 KHz at a center frequency of 5.8 GHz. The maximum frequencyerror of ±232 KHz corresponds to a phase shift of ±334° across the44-chip spreading sequence. The receiving station may evaluate threefrequency hypotheses in order to reduce the worst-case phase shift to±111°, which corresponds to a combining loss of up to 1.4 decibels (dB).These three frequency hypotheses are for the nominal frequency, +26.7ppm from the nominal frequency, and −26.7 ppm from the nominalfrequency.

For the embodiment shown in FIG. 6, the receiving station may performdetection for three frequency hypotheses, e.g., during call setup. Ifthe acquisition hardware can evaluate only one frequency hypothesis foreach packet transmission, then the receiving station may cycle throughthe three frequency hypotheses and may evaluate a different frequencyhypothesis for each packet transmission. The receiving station should beable to detect the preamble in at most three packet transmissions. Upondetecting a packet transmission, the receiving station determines thefrequency error and corrects for this frequency error for subsequentpacket transmissions. The receiving station should be able to detecteach subsequent packet transmission with high probability.

FIG. 6 shows a specific embodiment of a preamble for the range extensionmode. For this embodiment, the preamble is composed of three sequences:(1) a long/outer 64-bit pilot sequence, (2) an intermediate sequence offour binary values for each pilot bit, and (3) a short/inner 11-chipBarker sequence for each binary value. This preamble design isadvantageous since it uses the 11-chip Barker sequence as a basicbuilding block. Hence, other stations that perform coherent accumulationover the 11-chip Barker sequence can also detect this preamble andrecognize that the wireless medium is busy.

Various other preamble designs may also be used for the range extensionmode. In general, a preamble may be generated with any number ofsequences, and each sequence may be of any length. In an embodiment, thepreamble is composed of a single sequence of 1408 or more chips, where1408=128×11. In another embodiment, the preamble is composed of twosequences—a pilot sequence and a spreading sequence that is longer than11 chips. For example, a spreading sequence of 44 pseudo-random chipshaving good correlation properties may be used for the preamble. In yetanother embodiment, the preamble is composed of more than two sequences.

FIG. 7 shows a block diagram of a transmitting station 710 and areceiving station 750 in WLAN 100. Stations 710 and 750 may each be anaccess point or a user terminal. For simplicity, each station isequipped with a single antenna for the embodiment shown in FIG. 7.

At transmitting station 710, a transmit processor 730 receives trafficdata from a data source 720 and processes (e.g., encodes, interleaves,symbol maps, and spreads) the traffic data in accordance with a selecteddata rate. Transmit processor 730 also generates a preamble (e.g., asshown in FIG. 3 or 6), multiplexes the chips generated for traffic dataand the chips generated for the preamble, and provides output chips. Atransmitter (TMTR) 732 processes (e.g., converts to analog, amplifies,filters, and upconverts) the output chips and generates a modulatedsignal, which is transmitted via an antenna 734.

At receiving station 750, an antenna 752 receives the transmitted signaland provides a received signal to a receiver (RCVR) 754. Receiver 754processes and digitizes the received signal and provides input samplesto an acquisition processor 760. Acquisition processor 760 performsacquisition, detects for packet transmissions, determines and correctsfor frequency error, and provides despread symbols, as described below.A receive processor 770 processes the despread symbols in a mannercomplementary to the processing performed by transmit processor 730 andprovides decoded data to a data sink 772.

Controllers/processors 740 and 780 direct operation at transmittingstation 710 and receiving station 750, respectively. Memories 742 and782 store data and/or program codes for stations 710 and 750,respectively.

Receiver 754 provides complex-valued input samples at the sample rate,which is equal to or higher than the chip rate. For simplicity, thefollowing description assumes that the input samples are provided at thechip rate. For 802.11b, the chip rate is 11 Mcps, and the bit rate andsymbol rate are 1 Mbps for the preamble. Hence, a symbol period (T_(s))is 1 μs, and a chip period (T_(c)) is 90.9 nanoseconds (ns) for the802.11b preamble. For the preamble shown in FIG. 6, the bit rate andsymbol rate are 250 Kbps, and a symbol period (T_(s)) is 4 μs and covers44 chips. In the following description, “n” is an index for chip period,“k” is an index for frequency bin, and “i” is an index for the pilotbits in the preamble.

FIG. 8 shows a block diagram of an acquisition processor 760 a, which isan embodiment of acquisition processor 760 in FIG. 7. Within processor760 a, a multiplier 810 multiplies the input samples with a complexsinusoidal signal e^(j2π·f·n) and provides rotated samples. If theidentity of the transmitting station is not known, then the frequency ofthe sinusoidal signal is determined by a frequency hypothesis beingevaluated. If the identity of the transmitting station is known, thenthe frequency of the sinusoidal signal is determined by the frequencyerror between the transmitting and receiving stations.

A despreader 820 despreads the rotated samples and provides despreadsymbols. During acquisition, despreader 820 despreads the rotatedsamples over L chips and provides despread symbols at the chip rate,where L may be equal to 11, 22, 44, or some other value. For each chipperiod n, despreader 820 multiplies L input samples for chip periods nthrough n−L+1 with L chips of an L-chip spreading sequence, accumulatesthe L multiplication results, and provides a despread symbol x(n) forthat chip period. In an embodiment, L is equal to 11, the L-chipspreading sequence is the 11-chip Barker sequence, and despreader 820performs despreading over the length of the 11-chip Barker sequence. Inanother embodiment, L is equal to 44, the L-chip spreading sequence isthe 44-chip spreading sequence shown in FIG. 6, and despreader 820performs despreading over the length of the 44-chip spreading sequence.For other embodiments, L may be equal to other values, and other L-chipspreading sequences may be used for despreading. In any case, despreader820 performs a sliding correlation of the input samples with the L-chipspreading sequence to obtain a despread symbol for each chip period(instead of each symbol period) and provides L despread symbols for eachL-chip interval. These L despread symbols correspond to L differentpossible chip offsets (or L timing hypotheses) for the correct timing.

For the embodiment shown in FIG. 8, a unit 840 computes the squaredmagnitude of each despread symbol from despreader 820. In anotherembodiment that is not shown in FIG. 8, multiple despread symbols arecoherently accumulated, and unit 840 computes the squared magnitude ofeach coherently accumulated result. For both embodiments, an accumulator850 performs non-coherent accumulation for each different chip offset.If L=11, e.g., for the preamble shown in FIG. 3, then there are 11different chip offsets and accumulator 850 may accumulate the magnitudesquares of up to 128 despread symbols for each chip offset. If L=44,e.g., for the preamble shown in FIG. 6, then there are 44 different chipoffsets and accumulator 850 may accumulate the magnitude squares of upto 64 despread symbols for each chip offset. Accumulator 850 performs asliding non-coherent accumulation and, for each L-chip interval,provides L accumulated results for L different chip offsets.

A signal/preamble detector 870 receives the L accumulated results foreach L-chip interval, compares each accumulated result against athreshold S_(th), and declares the presence of a preamble if theaccumulated result exceeds the threshold. Signal/preamble detector 870continues to monitor the accumulated results to search for a peak valueand provides the chip offset for this peak value as the timing (tau) forthe detected preamble.

A symbol buffer 830 stores the despread symbols from despreader 820.Upon detection of the preamble, a frequency error estimator 880 receivesthe despread symbols from symbol buffer 830 and the timing (tau) fromsignal/preamble detector 870. Frequency error estimator 880 determinesthe frequency error in the detected preamble and provides a frequencyerror estimate.

FIG. 9A shows a block diagram of an acquisition processor 760 b, whichis another embodiment of acquisition processor 760 in FIG. 7. Withinprocessor 760 b, a multiplier 910 multiplies the input samples with acomplex sinusoidal signal and provides rotated samples. A despreader 920despreads the rotated samples with an L-chip spreading sequence andprovides L despread symbols for each L-chip interval. Multiplier 910 anddespreader 920 operate in the same manner as multiplier 810 anddespreader 820, respectively, in FIG. 8.

A delay multiplier 940 generates 1-symbol and 2-symbol delayed productsof the despread symbols, as described below. A 1-symbol delayed producty₁(n) is indicative of the phase difference between two despread symbolsx(n) and x(n−T_(s)) that are separated by one symbol period. A 2-symboldelayed product y₂(n) is indicative of the phase difference between twodespread symbols x(n) and x(n−2T_(s)) that are separated by two symbolperiods. A differential correlator 950 a receives the 1-symbol delayedproducts y₁(n), performs correlation between the 1-symbol delayedproducts and the expected values for these products, and provides acorrelation result c₁(n) for each chip period. Similarly, a differentialcorrelator 950 b receives the 2-symbol delayed products y₂(n), performscorrelation between the 2-symbol delayed products and the expectedvalues for these products, and provides a correlation result c₂(n) foreach chip period.

The phases of the correlation results c₂(n) from differential correlator950 b may not be aligned with the phases of the correspondingcorrelation results c₁(n) from differential correlator 950 a. Amultiplier 962 multiplies each correlation result c₂(n) fromdifferential correlator 950 b with a complex phasor e^(−jθ) ^(q) for Qdifferent hypothesized phases and provides a set of Q phase-rotatedcorrelation results for each chip period. For example, the hypothesizedphases may be {0, 60°, −60°} for Q=3, {0, 90°, 180°, −90°} for Q=4, andso on. The Q hypothesized phases may be selected to cover the possiblerange of relative phases. For example, the maximum phase differencebetween the 1-symbol and 2-symbol delayed correlations is approximately90 degrees for the maximum frequency error of ±232 KHz. Hence, if threehypothesized phases of 0, 60°, and −60° are used, then least onehypothesized phase is within 30°.

For each chip period n, an adder 964 coherently adds the correlationresult from differential correlator 950 a with each of the Qcorresponding phase-rotated correlation results from multiplier 962 andprovides Q combined correlation results z_(q)(n), for q=1, . . . , Q.For each chip period n, a unit 966 computes the squared magnitude ofeach of the Q combined correlation results, identifies the largestsquared magnitude value among the Q squared magnitude values, andprovides this largest squared magnitude value Z(n). For each chip periodn, a signal/preamble detector 970 compares the largest squared magnitudevalue Z(n) against a threshold Z_(th) and declares the presence of apreamble if Z(n) exceeds the threshold Z_(th). Signal/preamble detector970 continues to monitor the squared magnitude values to search for apeak value and provides the chip offset for this peak value as thetiming (tau) for the detected preamble.

A symbol buffer 930 stores the despread symbols from despreader 920. Afrequency error estimator 980 determines the frequency error in thedetected preamble and provides a frequency error estimate.

FIG. 9B shows an embodiment of delay multiplier 940 in FIG. 9A. Withindelay multiplier 940, the despread symbols x(n) are provided to twomultipliers 942 a and 942 b and also to two series-coupled delay units944 a and 944 b. Each delay unit 944 provides a delay of one symbolperiod T_(s), which is equal to 11 chip periods for L=11 and 44 chipperiods for L=44. Units 946 a and 946 b provide the complex conjugate ofthe despread symbols from delay units 944 a and 944 b, respectively.Multiplier 942 a multiplies the despread symbol x(n) for each chipperiod n with the output of unit 946 b and provides the 1-symbol delayedproduct y₁(n) for that chip period. Multiplier 942 b multiplies thedespread symbol for each chip period n with the output of unit 946 a andprovides the 2-symbol delayed product y₂(n) for that chip period.

FIG. 9C shows an embodiment of a differential correlator 950 m, whichmay be used for each of differential correlators 950 a and 950 b in FIG.9A. Within differential correlator 950 m, the m-symbol delayed productsy_(m)(n), for mε{1, 2}, are provided to a sequence of alternating delayunits 952 and 954. Each delay unit 952 provides a delay of one chipperiod, each delay unit 954 provides a delay of L−1 chip periods, andeach pair of delay units 952 and 954 provides a total delay of L chipperiods, which is one symbol period. Differential correlator 950 mincludes P delay units 952 and P−1 delay units 954. For 1-symbol delayeddifferential correlator 950 a, P is equal to 127 for the 802.11bpreamble shown in FIG. 3 and is equal to 63 for the preamble shown inFIG. 6. For 2-symbol delayed differential correlator 950 b, P is equalto 126 for the 802.11b preamble shown in FIG. 3 and is equal to 62 forthe preamble shown in FIG. 6. P is thus dependent on the number of bitsin the preamble (B) and the amount of delay (m), or P=B−m.

P adders 956 couple to P delay units 952. Each adder 956 sums the inputand output of an associated delay unit 952 and provides an output. Pmultipliers 958 couple to P adders 956 and also receive P expectedvalues a_(m,1) through a_(m,P) for the P m-symbol delayed products.Expected value a_(m,1) for m={1, 2} and i=1, . . . , P, may be computedas a_(1,i)=d_(i−1)·d_(i) for 1-symbol delayed differential correlator950 a and as a_(2,i)=d_(i−1)·d_(i+1) for 2-symbol delayed differentialcorrelator 950 b. Expected value a_(m,i) is then computed in the samemanner as the m-symbol delayed product, which is y_(m)(n)=x(n)·x*(n−m)However, because the pilot bits are real values, the complex conjugatemay be ignored for the expected values, e.g.,a_(1,i)=d_(i−1)·d_(i)*=d_(i−1)·d_(i). Each multiplier 958 multiplies theoutput of an associated summer 956 with its expected value a_(m,i). Foreach chip period n, an adder 960 sums the outputs from all P multipliers958 and provides a correlation result c_(m)(n) for that chip period.

FIG. 10 shows a block diagram of a frequency error estimator 880 a,which is an embodiment of frequency error estimator 880 in FIG. 8 andfrequency error estimator 980 in FIG. 9A. Frequency error estimator 880a receives from symbol buffer 830 or 930 N despread symbols that arespaced apart by L chip periods (or one symbol period) starting at thetiming tau provided by signal/preamble detector 870 or 970. The firstdespread symbol is thus time-aligned with the best timing hypothesis. Nmay be any integer value that is less than or equal to the number ofpilot bits in the preamble, e.g., N may be 32, 64, or 128. Withinfrequency error estimator 880 a, N multipliers 1012 receive the Ndespread symbols and N corresponding pilot bits in the preamble. Eachmultiplier 1012 multiplies its despread symbol with its pilot bit d_(i)to remove the modulation on that despread symbol. A unit 1014 receivesthe N outputs from N multipliers 1012, performs an N-point fast Fouriertransform (FFT) or discrete Fourier transform (DFT) on these N outputs,and provides N frequency-domain values for N frequency bins. N units1016 receive the N frequency-domain values from FFT/DFT unit 1014. Eachunit 1016 computes the squared magnitude of its frequency-domain valueand provides the detected energy for a respective frequency bin k.

After removing the modulation with multipliers 1012, the N outputs fromthese multipliers may have a periodic component. This periodic componentis caused by frequency error between the transmitting and receivingstations. FFT/DFT unit 1014 provides a spectral response of the Noutputs from multipliers 1012. The frequency bin with the largestdetected energy is indicative of the frequency error between thetransmitting and receiving stations.

A selector 1018 selects the largest detected energy among the N detectedenergies for the N frequency bins. A signal/preamble detector 1020compares the largest detected energy against a threshold E_(th),declares signal detection if the largest detected energy is greater thanthe threshold E_(th), and provides the frequency bin with the largestdetected energy as the frequency error estimate. The threshold E_(th)may be set equal to the total received energy for the preamble times ascaling factor. Signal/preamble detection may be performed in multiplestages (e.g., with detector 870 or 970 and detector 1020) to improvedetection performance.

In another embodiment of preamble detection and frequency errorestimation, the input samples are correlated with the pilot sequence fordifferent hypothesized frequency errors. For each hypothesized frequencyerror, the input samples are rotated by that frequency error, therotated samples are correlated with the pilot sequence, the correlationresult is compared against a threshold, and signal/preamble detection isdeclared if the correlation result exceeds the threshold. Thecorrelation may be performed in the time domain with a finite impulseresponse (FIR) filter structure or in the frequency domain with anFFT-multiply-IFFT operation. The frequency error estimate is given bythe hypothesized frequency error that yields the largest correlationresult exceeding the threshold.

In yet another embodiment of frequency error estimation, the inputsamples are initially despread to obtain despread symbols at chip rate,as shown in FIG. 8 or 9A. The despread symbols are then multiplied withthe corresponding pilot bits to remove the pilot modulation. Theresultant symbols are used to generate 1-symbol and 2-symbol delayedproducts, e.g., using delay multiplier 940 in FIG. 9B. The delayedproducts for each delay are processed to generate a complex value forthat delay. For each delay m, where m={1, 2}, the m-symbol delayedproducts are provided to L−1 series-coupled chip-spaced delay units toobtain m-symbol delayed products at L different chip offsets. For eachchip offset, the m-symbol delayed products for that chip error arecoherently accumulated across the preamble. The L accumulated resultsfor the L chip offsets may be combined (e.g., using maximal ratiocombining) to generate a complex value V_(m) for delay m. The phasedifference between the complex values V₁ and V₂ for 1-symbol and2-symbol delays may be computed and used to derive the frequency error.

The frequency error estimate derived based on any of the techniquesdescribed above typically contains residual frequency error. Thisresidual frequency error may be estimated by deriving a first L-tapchannel estimate based on the first half of the preamble and deriving asecond L-tap channel estimate based on the second half of the preamble,with both channel estimates being derived with the initial frequencyoffset estimate removed. The product of the second channel estimate andthe complex conjugate of the first channel estimate may be computed, ona per tap basis. The L resultant products may be coherently summed toobtain the phase difference between the two channel estimates.Thresholding may be performed on (1) each channel tap prior to computingthe product and/or (2) each product prior to summing the products. Thethresholding removes channel taps with low energy below a predeterminedthreshold. The residual frequency error may be estimated based on thephase difference between the two channel estimates and may be combinedwith the initial frequency error estimate to obtain a final frequencyerror estimate.

The frequency error between the transmit and receiving stations may beremoved by (1) adjusting the frequency of the downconversion LO signalwithin receiver 754 in FIG. 7 or (2) applying a sinusoidal signal withthe proper frequency (which is the negative of the frequency errorestimate) to multiplier 810 in FIG. 8 or multiplier 910 in FIG. 9A.Despreaders 820 and 920 may perform despreading over 11 or more chipsduring acquisition and over 11 chips during data reception.

FIG. 11 shows an embodiment of a process 1100 performed by a receivingstation to receive data. The receiving station rotates input sampleswith a sinusoidal signal to obtain rotated samples (block 1112). Duringinitial acquisition without knowledge of the transmitting station'sidentity, the sinusoidal signal has a frequency corresponding to ahypothesized frequency error between a transmitting station and thereceiving station. During subsequent acquisition with knowledge of thetransmitting station's identity as well as during data reception, thesinusoidal signal has a frequency corresponding to the estimatedfrequency error between the transmitting and receiving stations. Thereceiving station then despreads the rotated samples with an L-chipspreading sequence to obtain despread symbols, where L may be greaterthan 11 (e.g., L=44) during acquisition and may be equal to 11 duringdata reception (block 1114).

For acquisition, the receiving station detects for a preambletransmitted in the WLAN based on the despread symbols (block 1116). Thereceiving station may perform non-coherent accumulation on the despreadsymbols to obtain accumulation results and may detect for the preamblebased on the accumulated results, e.g., as shown in FIG. 8. Thereceiving station may also derive products of despread symbols for atleast two delays, perform correlation of the products for each delaywith the expected values for the delay, combine correlation results forthe at least two delays, and detect for the preamble based on thecombined correlation results, e.g., as shown in FIGS. 9A through 9C. Thereceiving station determines the frequency error between thetransmitting and receiving stations based on the detected preamble(block 1118). The receiving station may determine the energies of thedespread symbols for multiple frequency bins and may provide thefrequency bin with the largest detected energy as the frequency error,e.g., as shown in FIG. 10.

FIG. 12 shows an embodiment of an apparatus 1200 for receiving data in aWLAN. Apparatus 1200 includes at least one processor 1212 for rotatinginput samples with a sinusoidal signal to obtain rotated samples, atleast one processor 1214 for despreading the rotated samples with anL-chip spreading sequence to obtain despread symbols, where L>11 duringacquisition, at least one processor 1216 for detecting for a preambletransmitted in the WLAN based on the despread symbols, and at least oneprocessor 1218 for determining the frequency error between thetransmitting and receiving stations based on the detected preamble.

For clarity, various range extension techniques have been specificallydescribed for 802.11b/g. These techniques may also be used for otherIEEE 802.11 standards. For example, in 802.11a, a preamble is composedof 10 short training symbols and 2 long training symbols, where eachshort training symbol is composed of 16 complex-valued symbols. Thefrequency correction techniques may be used to (1) determine and correctfor the frequency error between the transmitting and receiving stationsand (2) perform coherent accumulation over more than 16 complex-valuedsymbols, which may improve detection performance.

The range extension techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, firmware, software, or a combination thereof. For a hardwareimplementation, the processing units at a receiving station may beimplemented within one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, or a combination thereof. Theprocessing units at a transmitting station may also be implementedwithin one or more ASICs, DSPs, processors, and so on.

For a firmware and/or software implementation, the techniques may beimplemented with codes (e.g., procedures, functions, instructions and soon) that may be used by at least one processor perform the functionsdescribed herein. The software codes may be stored in a memory (e.g.,memory 742 or 782 in FIG. 7) and executed by a processor (e.g.,processor 740 or 780). The memory may be implemented within theprocessor or external to the processor.

Further, for software implementations, the codes may be stored on ortransmitted over or stored on a computer-readable medium.Computer-readable media includes both computer storage media andcommunication media including any medium that facilitates transfer of acomputer program from one place to another. A storage media may be anyavailable media that can be accessed by a general purpose or specialpurpose computer. By way of example, and not limitation, suchcomputer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, or any other medium that can be used to carry or store desiredprogram code means in the form of instructions or data structures andthat can be accessed by a general-purpose or special-purpose computer,or a general-purpose or special-purpose processor. Also, any connectionis properly termed a computer-readable medium. For example, if thesoftware is transmitted from a website, server, or other remote sourceusing a coaxial cable, fiber optic cable, twisted pair, digitalsubscriber line (DSL), or wireless technologies such as infrared, radio,and microwave, then the coaxial cable, fiber optic cable, twisted pair,DSL, or wireless technologies such as infrared, radio, and microwave areincluded in the definition of medium. Disk and disc, as used herein,includes compact disc (CD), laser disc, optical disc, digital versatiledisc (DVD), floppy disk and blu-ray disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers.Combinations of the above should also be included within the scope ofcomputer-readable media.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. An apparatus comprising: at least one processor configured to performdetection of at least one initial packet transmission sent from a firststation to a second station in a wireless local area network (WLAN) withat least one frequency hypothesis from a plurality of frequencyhypotheses, to determine frequency error between the first and secondstations based on the at least one initial packet transmission, and toperform detection for at least one subsequent packet transmission fromthe first station with the frequency error corrected, wherein each ofthe plurality of frequency hypotheses corresponds to a differenthypothesized frequency error between the first and second stations, andwherein the at least one frequency hypothesis is selected such thatcoherent accumulation may be performed over an interval selected for arequired signal-to-noise-and-interference ratio (SNR) for the initialpacket transmission; and a memory coupled to the at least one processor.2. The apparatus of claim 1, wherein the at least one processor isconfigured to perform coherent accumulation over more than 11 chips todetect for the at least one initial packet transmission.
 3. Theapparatus of claim 1, wherein the at least one processor is configuredto perform digital rotation on samples to correct the frequency errorbetween the first and second stations.
 4. The apparatus of claim 1,further comprising: a receiver configured to perform downconversion of aradio frequency (RF) input signal with a downconversion signal, whereinfrequency of the downconversion signal is adjusted to correct thefrequency error between the first and second stations.
 5. The apparatusof claim 1, wherein the at least one subsequent packet transmission isscheduled.
 6. The apparatus of claim 1, wherein the at least one initialpacket transmission and the at least one subsequent packet transmissionare sent at a data rate that is lower than 1 mega bits/second (Mbps). 7.The apparatus of claim 1, wherein the required SNR for the initialpacket transmission corresponds to a range extension mode.
 8. The methodof claim 1, wherein the required SNR for the initial packet transmissioncorresponds to a range extension mode.
 9. A method comprising:performing detection for at least one initial packet transmission sentfrom a first station to a second station in a wireless local areanetwork (WLAN) with at least one frequency hypothesis from a pluralityof frequency hypotheses, wherein each of the plurality of frequencyhypotheses corresponds to a different hypothesized frequency errorbetween the first and second stations, and wherein the at least onefrequency hypothesis is selected such that coherent accumulation may beperformed over an interval selected for a requiredsignal-to-noise-and-interference ratio (SNR) for the initial packettransmission; determining frequency error between the first and secondstations based on the at least one initial packet transmission; andperforming detection for at least one subsequent packet transmissionfrom the first station with the frequency error corrected.
 10. Themethod of claim 9, further comprising: performing digital rotation oninput samples to correct the frequency error between the first andsecond stations.
 11. The method of claim 9, wherein the at least onesubsequent packet transmission is scheduled.
 12. An apparatuscomprising: means for performing detection for at least one initialpacket transmission sent from a first station to a second station in awireless local area network (WLAN), with at least one frequencyhypothesis from a plurality of frequency hypotheses, wherein each of theplurality of frequency hypotheses corresponds to a differenthypothesized frequency error between the first and second stations, andwherein the at least one frequency hypothesis is selected such thatcoherent accumulation may be performed over an interval selected for arequired signal-to-noise-and-interference ratio (SNR) for the initialpacket transmission; means for determining frequency error between thefirst and second stations based on the at least one initial packettransmission; and means for performing detection for at least onesubsequent packet transmission from the first station with the frequencyerror corrected.
 13. The apparatus of claim 12, further comprising:means for performing digital rotation on input samples to correct thefrequency error between the first and second stations.
 14. The apparatusof claim 12, wherein the required SNR for the initial packettransmission corresponds to a range extension mode.
 15. A non-transitorycomputer readable medium comprising: code for causing at least oneprocessor to detect at least one initial packet transmission sent from afirst station to a second station in a wireless local area network(WLAN) with at least one frequency hypothesis from a plurality offrequency hypotheses, wherein each of the plurality of frequencyhypotheses corresponds to a different hypothesized frequency errorbetween the first and second stations, and wherein the at least onefrequency hypothesis is selected such that coherent accumulation may beperformed over an interval selected for a requiredsignal-to-noise-and-interference ratio (SNR) for the initial packettransmission; code for causing at least one processor to determinefrequency error between the first and second stations based on the atleast one initial packet transmission; and code for causing at least oneprocessor to detect at least one subsequent packet transmission from thefirst station with the frequency error corrected.
 16. The computerreadable medium of claim 15, further comprising: code for causing atleast one processor to perform digital rotation on input samples tocorrect the frequency error between the first and second stations. 17.The computer readable medium of claim 15, wherein the required SNR forthe initial packet transmission corresponds to a range extension mode.